Optoelectronic measurement systems are useful for determining various optical properties of optical systems, subsystems, components or samples under test. In general, light reflected from, transmitted through or generated by a unit under test (UUT) falls on a photodetecting device which then transduces the light into an electrical signal. This electrical signal is then analyzed to obtain various optical parameters associated with the UUT. Myriad types of analyses can be accomplished with these optoelectronic measurement systems to yield highly useful information about the UUT. In general, any type of light can be analyzed by optoelectronic measurement systems, for example, white light, monochromatic light, polarized light or unpolarized light.
Optoelectronic systems which make measurements of a UUT generally comprise optical receivers that focus light from the UUT onto a photodetector. One important optical receiver parameter is the "optical return loss" (ORL) defined as: EQU ORL=10 log (P.sub.i /P.sub.r);
where P.sub.r is the reflected optical power from the receiver and P.sub.i is the incident optical power to the receiver. The receiver ORL is thus a measure of the incident optical power to the receiver as compared to the reflected optical power from the receiver.
It is critical that the ORL be maximized. For example, reflection from the receiver must be minimized to avoid perturbing other sensitive elements of an optical system under test, such as a laser. Linewidth variations from an indium gallium arsenide phosphide (InGaAsP) distributed feedback laser have been observed for reflected optical feedback as low as 0.000001%. See R. W. Tkach and A. R. Chraplyvy, "Linewidth Broadening and Mode Splitting Due to Weak Feedback in Single Frequency 1.5 Micrometer Lasers," Elect. Lett. 21, 1081 (1985) and R. W. Tkach and A. R. Chraplyvy, "Regimes of Feedback Effects in 1.5 Micrometer Distributed Feedback Lasers," J. Lightwave Technol., LT-4, 1655 (1986). Thus, even relatively minor reflections from the receiver can perturb sensitive optical elements. Great effort has bee devoted in the art to increasing the receiver ORL in optoelectronic measurement systems to alleviate these problems.
A major cause of degraded receiver ORL occurs when light incident to the photodetector utilized in the receiver reflects within the numerical aperture of the optical focusing means. One approach to increasing the ORL in this situation is to coat the photodetector surface with an antireflection (AR) coating, thereby decreasing the level of reflected light from the photodetector. Single and multiple layer antireflection coatings have been used with homogeneous photodetector materials, such as silicon or germanium. For example, it is known that for a homogeneous material, a single antireflection coating having a refractive index equal to the geometric mean of the refractive index of the homogeneous material and the incident medium, and an optical thickness equal to a quarter wavelength of the incident light will cause the reflected optical power to be zero at the center wavelength.
However for planar InP/InGaAs/InP PIN heterojunction material structures, the implementation of an antireflection coating is limited by the thickness accuracy of the organometallic vapor phase epitaxy grown InP and InGaAs layers. See, e.g., D. M. Braun, "Design of Single Layer Antireflection Coatings for InP/In.sub.0.53 Ga.sub.0.47 As/InP Photodetectors for the 1200-1600 nm Wavelength Range," Appl. Opt. 27, 2006 (1988). Antireflection coatings disclosed in this article experimentally achieved 0.49% reflected optical power at a wavelength of 1312 nm and 0.20% reflected optical power at a wavelength of 1515 nm. While AR coatings increase ORL, optoelectronic measurement systems with optical elements having high sensitivities cannot tolerate an ORL of this magnitude. Thus, planar AR coated surfaces do not satisfy the need for maximizing the ORL of receivers employing heterostructure photodetectors in sensitive optoelectronic measurement systems.
Another approach to improving the OR of optical receivers is the use of low reflectivity surface topographies. Known low reflectivity surface topographies in the form of grooves have typically been fabricated on homogeneous, uniform materials. Grooved surfaces of this nature, referred to as "surface relief gratings," are disclosed in M. G. Moharam and T. K. Gaylord, "Diffraction Analysis of Dielectric Surface Relief Gratings," J. Opt. Soc. Am. 72, 1385 (1982); Erratum 73, 411 (1983); T. K. Gaylord, W. E. Baird and M. G. Moharam, "Zero Reflectivity High Spatial-Frequency Rectangular-Groove Dielectric Surface Relief Gratings," Appl. Opt. 25, 4562 (1986); R. C. Enger and S. K. Case, "Optical Elements with Ultrahigh Spatial-Frequencies Surface Corrugations," Appl. Opt. 22, 3220 (1983). Furthermore, it is known that dielectric surface relief gratings exhibit good antireflective properties and very low diffracting efficiency in backward diffracted orders. See, e.g., T. K. Gaylord and M. G. Moharam, "Analysis and Applications of Optical Diffraction by Gratings," Pro. IEEE. 73, 894 (1985).
However, the surface relief gratings disclosed in the aforementioned articles require that the optical repeat distance of the grooves. .LAMBDA., be less than or equal to the wavelength of the incident light in free space, .lambda.. Under this condition, the ORL generally improves with increased groove depth. The aforementioned Enger and Case article suggests that these high aspect ratio and high spatial frequency grooves act as a surface whose average index of refraction is smoothly tapered from that of air to that of the substrate material. Therefore, in heterostructure photodetectors having different layer refractive indices and with the grooves formed in the top layer, the reflected power from the layer interfaces will efficiently couple to the zero order backward diffracted wave. These layer interface reflections can be significant. For example, the magnitude of reflection from a planar InP/InGaAs interface for normal incident light with a wavelength of 1300 nm is theoretically calculated to be about 0.4%. Thus, these designs efficiently couple the large interface reflection to the zero order backward diffracted wave and are not conducive to limiting the ORL in optical receivers employing heterostructure photodetectors used in optoelectronic measurement systems. Deep grooves are also inappropriate for small area PIN photodetectors since a very thick p layer is required to accommodate the grooves. The growth time for the p layer will cause the p and n type dopants to diffuse into the i layer causing performance degradation. Therefore, known topographies with grooves having .LAMBDA. .ltoreq..lambda. cannot efficiently increase receiver ORL in photodetectors to achieve low reflected power.